1. Field of the Invention
The present invention relates to an impeller for a water pump that maintains a discharge performance by making a discharge flow rate constant in a low rotation region of an engine rotation speed and reduces the discharge flow rate in a high rotation region. Note that in this specification, a discharge flow is a cooling water flow actually discharged by the water pump per unit time, and the discharge performance is the discharge flow per unit rotation speed.
2. Description of the Related Art
In recent years, demand for reduced fuel consumption in vehicles has risen. In accordance therewith, demand for improved efficiency in respective constitutional components installed in vehicles is gradually rising. Of the constitutional components installed in a vehicle, a water pump is a component having an important function for maintaining an optimum temperature by adjusting temperatures of an engine, various electronic circuits, a heater core, and so on. Water pumps can be broadly divided into two types, namely a mechanically driven water pump and an electrically driven water pump. In recent years, use of electrically driven water pumps has increased gradually, but due to the high cost thereof, proportionally more mechanically driven water pumps remain in use at present.
An amount of circulating cooling water required to cool the engine, when considered as a ratio relative to an engine rotation speed, typically tends to increase in low and intermediate rotation regions in comparison with a high rotation region. This is in order to suppress knocking in the low and intermediate rotation regions. Therefore, although an absolute value is small in a low engine rotation speed region, a proportionally larger cooling water flow is required.
Conversely, although the absolute value is large in a high engine rotation speed region, the required cooling water flow does not increase proportionally. Further, when the cooling water flow is increased excessively, cavitation may occur. However, the discharge flow of the water pump is normally proportionate to the rotation speed. A water pump that responds to the required cooling water flow is disclosed in Japanese Patent Application Publication No. H7-208393. In the water pump disclosed in Japanese Patent Application Publication No. H7-208393, a central portion of an impeller serves as a plate spring support portion 25, deforming plate springs 24 are disposed on an impeller inner peripheral side for respective vanes, one end of the plate spring 24 is supported by a vane 13, another end of the plate spring 24 is supported by the plate spring support portion 25, and the vanes 13, which are structures that move but do not deform, are disposed on an impeller outer peripheral side.
As the impeller of the water pump rotates more quickly, a rotation speed of the vanes 13 also increases, and as a result, a water pressure force acting on the vanes 13 increases such that the vanes 13 rock in an opposite direction to a rotation direction. More specifically, the vanes 13 move from a solid line to a dotted line in FIG. 2 of Japanese Patent Application Publication No. H7-208393. Accordingly, an outer diameter of the vanes 13 decreases steadily as the impeller of the water pump rotates more quickly, and therefore, although the absolute value of the discharge flow of the water pump is large, the discharge flow decreases proportionally to the rotation speed. By performing this control, a cooling water flow is secured in the low engine rotation speed region, while in the high engine rotation speed region, unnecessary use is eliminated and cavitation is suppressed. Note that in this specification, the outer diameter of the vane denotes an outer diameter of a virtual circle traveling around an outermost periphery of the vane.
However, the following problem remains in the water pump disclosed in Japanese Patent Application Publication No. H7-208393. An amount of rocking in the vanes 13 of the water pump when the vanes 13 receive the water pressure force is determined by a balance between the water pressure force and a force of the plate springs 24. However, irregularities invariably occur in the individual characteristics and shapes of the plate springs 24 and the vanes 13, and therefore the amount of rocking (a tilt angle) in the vanes 13 varies among the respective vanes 13. When the amount of rocking in the vanes 13 varies among the respective vanes 13, the water pressure force acting on the vanes 13 becomes more irregular, leading to further irregularities in the rocking amount of the vanes 13, and thus a vicious circle is established. When the rocking amount of the vanes 13 varies in this manner, the discharge performance of the water pump also varies, and as a result, it becomes difficult to secure a desired discharge performance.
A water pump disclosed in Japanese Patent Application Publication No. H10-122177 may be cited as a constitution in which movement amounts of the respective vanes are made even, although here the impeller is moved in accordance with a water temperature rather than the water pressure force, as in the water pump disclosed in Japanese Patent Application Publication No. H7-208393. In a water pump 2 disclosed in Japanese Patent Application Publication No. H10-122177, linear slits 16 are provided in a single movable plate 13 in an identical number to the number of vanes, and a pump impeller 14 formed by engaging a pin 14a with the linear slit 16 is supported to be capable of moving within the linear slit 16 and revolving about the pin 14a. Further, a bimetal (a thermo-sensitive drive source) 15 is disposed upstream and in the center of the pump impeller 14, and the bimetal 15 applies a spring force to the movable plate 13 by deforming in accordance with a cooling water temperature.
As the cooling water temperature rises and falls, the bimetal 15 expands and contracts, and accordingly, the movable plate 13 rotates in a rotation direction. The pump impeller 14 is connected to the movable plate 13, and therefore, when the movable plate 13 rotates, the pump impeller 14 also moves. More specifically, when the cooling water temperature is high, the pump impeller 14 is moved outwardly in a radial direction such that an outer diameter of the pump impeller 14 increases, and as a result, the discharge performance of the water pump increases. When the cooling water temperature is low, the pump impeller 14 is moved to an inner peripheral side such that the outer diameter of the pump impeller 14 decreases, and as a result, the discharge performance of the water pump decreases.
Thus, overheating is suppressed when the temperature of the cooling water is high, and unnecessary use is reduced when the temperature of the cooling water is low. However, with the water pump constitution disclosed in Japanese Patent Application Publication No. H10-122177, all of the pump impellers 14 are driven to rotate by the bimetal (the thermo-sensitive drive source) 15, and therefore the vanes cannot be driven in accordance with the rotation speed, as in the water pump disclosed in Japanese Patent Application Publication No. H7-208393. Further, the bimetal 15 is disposed upstream of the pump impeller 14 so as to project to the outside of the pump impeller 14, and therefore a flow of the cooling water may be disturbed by the bimetal 15, causing the discharge performance to decrease, and as a result, cavitation may occur. Furthermore, the bimetal may not respond favorably to the constantly changing temperature of the cooling water. Hence, in the high engine rotation speed region, reductions may occur in the discharge performance and efficiency. In FIG. 1 of Japanese Patent Application Publication No. H10-122177 in particular, the bimetal 15 is axially supported by a rotary shaft 11. However, the rotary shaft 11 of the water pump is typically made of bearing steel in order to secure sufficient strength and is therefore extremely hard. As a result, as shown in FIG. 1, processing for providing a shaft on a tip end of the rotary shaft 11 requires time and money.